Tubulars used to drill and complete bore holes in earth materials are typically joined by threaded connections. Numerous threaded connection geometries are employed to provide sealing and load carrying capacities to meet drilling, installation and operating requirements. Of these geometries, tapered pipe threads are among the simplest and most widely used.
Within the context of petroleum drilling and well completion, wells are typically constructed by drilling the well bore using one tubular string, largely comprised of drill pipe, then removing the drill pipe string and completing by installing a second tubular string, referred to as casing, which is subsequently permanently cemented in place. The tubular strings are formed by connecting lengths of pipe, referred to as joints, with threaded connections. With this historic method of well construction, both the drill pipe and casing joint designs are separately optimised for the different performance requirements of the drilling and completion operations respectively. More specifically, the drill pipe connections must accommodate torque required to drill, which is not required during completion.
Recent advances in drilling technology have enabled wells to be drilled and completed with a single casing string, eliminating the need to ‘trip’ the drill pipe in and out of the hole to service the bit and make room for the casing upon completion of drilling. This change is motivated by potential cost savings arising from reduced drilling time and the expense of providing and maintaining the drill string, plus various technical advantages, such as reduced risk of well caving before installation of the casing.
However, using casing to both drill and complete the well changes the performance requirements of the casing string, and more particularly the torque capacity of the casing connections, from those established through use within the historic methods of well construction.
The most widely used of casing connections, are the industry standard threaded and coupled buttress (BTC) and 8-round (LTC or STC) connections having tapered pipe thread geometries specified by the American Petroleum Institute (API). These connections have limited torque capacity and are thus not well suited to the casing drilling application, but are readily available and relatively inexpensive. To more fully realize the potential benefit of this emerging casing drilling system (CDS) technology, it is therefore desirable to find means to press these industry standard connections into service by identifying means to inexpensively increase their torque capacity.
Similar motivations to improve the sealing capacity of connections using API thread forms, have led to the invention of apparatus and methods such as described in U.S. patents, U.S. Pat. No. 4,706,997, U.S. Pat. No. 4,878,285, U.S. Pat. No. 5,283,748, U.S. Pat. No. 5,689,871 and U.S. Pat. No. 4,679,831. These patents generally describe inventions where a modified coupling, provided with an internal floating sleeve or seal ring, is employed to join pipes having standard API thread forms on their pin ends. The seal ring is positioned in the so-called J-section space between the pin ends of a made-up threaded and coupled connection. The seal ring internal diameter is approximately matched to the internal pipe diameter and is coaxially placed inside the coupling at its mid plane so as to engage both the pin ends when the connection is made up. According to the teachings of these inventions, this engagement or shouldering is primarily intended to enhance the seal performance of the connection beyond that provided by the standard API configuration. Several additional benefits are also obtained such as improved flow performance and a smooth running bore. The use of resilient materials in conjunction with the rigid seal ring or as separate seals are also taught as means to further promote sealing.
While these descriptions of the prior art do not explicitly address the utility of such a “convertible metal ring” or seal ring as a means to improve the torque capacity, otherwise available from API connections, the increased torque capacity is a well know benefit. In fact, manufacturers of such connections quantify this parameter in published performance data such as provided by Hunting Oilfield Services for a product described as “the KC Convertible coupling system”.
These prior art implementations of rigid seal rings recognise that the wide tolerance variation allowed for the pin and box geometries of threaded and coupled connections meeting API specifications permits a correspondingly wide range of axial position after make up, if a satisfactory level of interference or “dimensional control” is to be achieved (see U.S. Pat. No. 5,283,748). Consequently, to obtain satisfactory “dimensional control” this prior art teaches that additional measures must be taken to reduce the tolerance range of pins and/or boxes provided for use with seal rings and to control the make up position. Such steps include specifically manufacturing “modified boxes” to tighter tolerances than required by API specifications and pre-screening of product manufactured to API tolerances to similarly obtain pins and boxes having more precisely controlled geometry. To ensure controlled placement and retention of the seal ring, it is taught that additional machining of the coupling central thread region is required to form a seat for the seal ring. To obtain dimensional control of the so-called mill end make up position, additional fixtures or measurements are required.
However, these prior art couplings require modification of the standard API components or increased quality control and, therefore, substantially reduce the benefits of low cost and simplicity originally sought from using existing industry standard couplings and pins. In addition, prior art couplings are in large part motivated by the desire to upgrade the pressure containment capacity of API connections and, as such, are not optimised to obtain upgraded torque capacity desired for casing drilling applications.